In an interferometric fiber optic gyroscope a clockwise (CW) beam and a counterclockwise (CCW) beam pass around a coil of optical fiber and are brought together on leaving the coil to interfere on a detector. Rotation applied to the rate sensor induces a rate-related non-reciprocal phase shaft between the CW and CCW beams which causes the intensity on the detector to very co-sinusoidally with applied rate.
In general, two different methods of signal processing may be used. In closed loop systems, a compensatory non-reciprocal phase shift is applied to null the phase shift induced by rotation. In open loop systems, the intensity of the combined beams at the detector is used to determine the applied rate. In both methods, the measurand is linearly related to rate with the constant of proportionality being referred to as the Scale Factor.
Previously it has been thought necessary to use a closed loop architecture to achieve high accuracy of the scale factor. In this context, high accuracy means a scale factor performance which does not differ by more than 100 ppm with respect to an ideal output over a rate range of +/-500 deg/sec or greater, which is the rate range needed for aircraft and missile applications. Such an accuracy is needed for inertial navigation in both civil and military applications, and for guidance of long range missiles.
For agile military aircraft or naval vessels, where typically a ring laser gyroscope system is currently used, an even higher scale factor accuracy of about 5 ppm over the rate range is needed, and this represents one of the highest accuracy scale factor requirements.
To allow a rate sensor to replace the more expensive ring laser gyroscope for these applications, scale factor performance at this level is required. Replacement of ring laser gyroscopes by rate sensors is likely to produce a much lower cost navigation system, as the cost of the gyroscope sensors is one of the largest elements in the cost of the overall system.
For a fiber optic sensor with closed loop architecture, optical modulation is applied so that at zero rate there is a null signal. At rate, after demodulation, a signal is derived which is proportional to rate. This is used to operate a feedback loop as the error signal, and a second signal is applied to a phase modulator to null out this error signal. In a typical example, the modulation comprises a square wave operating at the correct frequency for the fiber coil. This is the frequency that causes a phase shift of 180.degree. between the two directions when the phase modulator is placed at one end of the coil, after a circuit of the coil and recombination, and is given by 1/(2*loop transit time). The loop transit time is the time taken for light to propagate from one end of the coil to the other. The feedback signal is typically a serrodyne ramp applied to a phase modulator in the optical circuit. This comprises a linear phase modulation ramp going from 0 radians to 2.pi. radians in a time t and then being reset to zero quickly, with the process then restarted. If the top of the ramp is exactly 2.pi. radians, this corresponds to a frequency shift of 1/t Hz, and this frequency shift is that needed to null out the rate signal, and the frequency is then the rate output. This signal is normally applied at the other end of the sensing coil to the square wave modulation, to null out the applied rate. In this case, the output of the gyroscope is then the frequency of the serrodyne ramp which is proportional to rate. This gives very good scale factor performance over a broad rate range but suffers from the following disadvantages:
(a) The light around the loop is at a different frequency in the CW and CCW directions, so that the gyroscope is fundamentally non reciprocal, and thus the gyroscope bias performance may be degraded. PA1 (b) It is necessary to implement a very accurate servo loop to hold the serrodyne reset amplitude at exactly 2.pi. radians. Methods of doing this have been devised, but there are problems in implementation at low rates when there is not a lot of information to drive the servo. In one implementation, the signal to drive the servo is obtained at the flybacks which occur seldom at very low rates. PA1 (c) Any feedback loop which has the attributes of gain and feedback may suffer from lock-in behaviour at low rate. This causes extreme scale factor errors in a rate range around zero rate so that sensitivity may be severely affected or completely lost. PA1 means defining a coil or ring around a sensing axis and around which light may propagate in a clockwise (CW) and a counterclockwise (CCW) direction; PA1 beam input means for introducing into the coil or ring a clockwise (CW) beam and a counterclockwise (CCW) beam to propagate in opposite directions around the coil or ring; PA1 means for combining the CW and CCW beams after passage around the coil or ring: PA1 detector means for detecting the intensity of the combined CW and CCW beams, PA1 phase modulator means for applying between the CW and CCW beams a plurality of different phase shifts, and processor means for monitoring the corresponding detected intensity for each of the phase shifts, thereby to sample the intensity across at least a major portion of a complete fringe. PA1 (i) multiplies the sum data by a cosine function and integrates to give a first signal, PA1 (ii) multiplies the sum data by a sine function and integrates to give a second signal, PA1 (iii) multiplies the difference data by a cosine function and integrates to give a third signal, and PA1 (iv) multiplies the difference data by a sine function and integrates to give a fourth signal,
By contrast, the conventional open loop gyroscope schemes are believed to offer good gyroscope drift performance due to the high degree of reciprocity, but generally poor scale factor performance due to the difficulty of accurately following the cosine wave curve of light output against rate to very high precision.
In our earlier U.K. Patent Application No. 9304016.0, which corresponds to U.S. Pat. No. 5,459,575, I describe a signal processing technique which involves the extraction of three signals at frequencies 1f,2f and 4f (with a modulation frequency of 1f) from the photodetector signal to account for three unknowns: the angular rate (i.e. the desired output); the intensity of light on the photodetector, and the amplitude of the phase modulation.
This compensation is needed as the intensity of light and the amplitude of the phase modulator are both subject to change due to external perturbations such as temperature and time and these parameters enter into the equation determining the rate. In our earlier technique, two ratios are formed from the three signals to eliminate the intensity effects, end the inverse tangent is taken of one ratio (1f/2f) to extract the rate, and the other ratio (2f/4f) is used to determine the amplitude of phase modulation.
This is a fairly complicated scheme, and requires three signal channels at the three frequencies which need to be very closely matched so that the ratios are meaningful. It is an aim of this invention to provide an optical gyroscope which obviates at least some of the above disadvantages.